In a case where a coarse surface such as a sheet of paper and a wall is irradiated with coherent light such as laser light, and reflected light or transmitted light thereof is observed, a light-dark spot pattern can be found. Such a spot pattern is referred to, for example, as a speckle pattern, simply as a speckle, or as a speckle image. In addition, a phenomenon of producing such a pattern is called, for example, a speckle phenomenon. The study of a property of the speckle phenomenon was started in earnest immediately after the advent of the laser.
Patent Document 1 discloses acquisition of a sound signal from vibrations of a speckle image caused by reflected light of laser light radiated to an object.